Components such as those used in the aerospace industry, and the like, are typically subjected to high stresses and high temperatures. The materials from which these components are made often dictate the operating limits for the apparatus in which they are employed. Extensive efforts have been made over the years to develop new alloys which permit operation of these components at higher operating temperatures and/or which lead to lighter weight, longer lived components. One group of alloys which has been found to be useful in fabricating these components are the titanium based alloys. These alloys are generally light weight and exhibit high temperature durability. However, due in part to this high temperature durability, fabrication of components from titanium based alloys presents problems for the manufacturer.
One common method of manufacturing metallic components is brazing, in which two or more metal parts are joined by applying heat and a brazing filler metal. The filler metal used has a melting temperature below the melting temperature of the metal parts being joined. When the metal parts are heated to a temperature above the melting temperature of the brazing filler metal the brazing filler metal flows by capillary action into gaps between the metal parts and joins them by creating a metallurgical bond between them at the atomic level. The brazing process is similar to soldering, but differs in that the filler metal is of greater strength, and has a higher melting temperature than soldering material. When properly designed a brazed joint will yield a high degree of serviceability under concentrated stress, vibration and temperature loads. Brazing is, therefore, a highly desirable method of joining metal parts used in aerospace components.
Common to the brazing of most metal parts is the problem of maintaining the parts in fixed relation to each other throughout the brazing cycle. Although parts may be oriented correctly when first placed on the surface which is to support them during brazing, differences in thermal expansion rates between the parts and the surface tend to distort and shift the parts with respect to each other. As a result, the parts may require significant post-braze machining or other operations to bring the parts within allowable tolerances. Brazing fixtures, such as those disclosed in U.S. Pat. Nos. 2,326,430; 2,614,517; 2,944,504; 3,094,957 and 4,212,690 which are incorporated herein by reference, attempt to reduce distortion and shifting of parts either by applying force to the parts to hold them rigidly in place during the brazing cycle, or by allowing the parts to slide over the surface which supports them during the brazing cycle. However, due to problems encountered in brazing large titanium alloy parts, neither of these characteristics is desirable in a fixture for brazing such parts.
In order to successfully braze titanium alloy parts, the parts must be held in fixed relation to each other at high temperature for an extended period of time. Typically, the brazing of large titanium parts may require maintaining temperatures exceeding 1700.degree. F. for in excess of four hours. At brazing temperature, the titanium alloy parts have minimal creep strength, and applying force to the parts to maintain them in a specific position could distort the shape of the individual parts, requiring substantial post-braze machining. Additionally, the thermal stresses which may build up in the parts due to differences in thermal expansion between the parts and the surface on which they are supposed to slide can cause the parts to shift with respect to each other during the brazing cycle, introducing distortion which requires post-braze machining to correct. Likewise those thermal stresses may remain in the parts after the brazing cycle is complete, requiring further treatment to relieve the stress.
In order to maintain large titanium parts in fixed relation to each other throughout the brazing process, the parts must be supported by a fixture which will not exert excessive force on the parts being brazed, and which exhibits thermal expansion essentially equal to that of the titanium parts being brazed so that the parts do not have to slide with respect to the surface which supports them. Ideally, a fixture would be made of the same material as the titanium alloy parts being brazed, so that thermal expansion between the parts and the fixture would be matched, and the parts would not shift. However, as mentioned above, at the temperature required to braze titanium alloy components, a fixture made of the titanium alloy has minimal creep strength and would tend to distort under the weight of the parts if not supported, shifting the position of the parts despite thermal matching of the parts and the support surface. It is, therefore, an object of the present invention to provide a brazing fixture which matches the thermal expansion of the titanium alloy parts, and resists creep at braze temperature, thereby maintaining the titanium alloy parts in fixed relation to each other throughout the brazing cycle.